Caves and arches: Waves eat away the rock at sea level, forming caves on each side of the headland. The caves eventually meet up, forming an arch.

Stacks form: Over time the arch collapses into the sea, leaving a rock ‘stack’.

Further undercutting: Further undercutting by waves, and opening up of vertical cracks in the rock by rain and saltwater, gradually reduce the stack to a low platform or reef. This occurred spectacularly in 2005, making world-wide news.